A wafer, also referred to as a slice or substrate, is a thin semiconductor material used in electronics for the fabrication of integrated circuits and in photovoltaics for wafer-based solar cells. The wafer serves as the substrate for microelectronic devices built in and over the wafer and undergoes many microfabrication process steps such as doping or ion implantation, etching, deposition of various materials, and photolithographic patterning. Finally individual microcircuits, such as semiconductor chips, are separated and packaged.
There are a number of factors that can affect the shape of a wafer. While a wafer is at full thickness, it typically has the tensile strength to resist any external influences from changing its shape. However, as a wafer is thinned, external influences will cause a wafer to become concave or convex. Some of the more common influences are film type and thickness of the wafer. For example, nitrides can cause a wafer to become concave while oxides can cause a wafer to become convex. Additionally, how heavily a wafer is doped can result in a potato chip shaped wafer. One cause of wafer curvature is a result of stress or strain caused by thermal cycling of the wafer during its fabrication and/or a mismatch of respective Coefficient of Thermal Expansion (CTE) values of the particular film and the wafer material. Generally, the amount of wafer curvature increases as the wafer becomes thinner. Therefore, preventing wafer curvature is particularly important in thin wafer applications.